The present invention generally relates to the field of acoustic, vibration, and shock motion reduction, and more particularly concerns a mount for reducing the acoustic, vibration, and shock forces transmitted between a mounted component and a support structure.
It is often desirable to mount equipment that generates acoustic and vibration forces to reduce the transmission of these forces into a supporting structure. Mounts typically used for acoustic and vibration isolation include elastomeric, metal spring, and air or gas mounts.
In certain cases these acoustic and vibration mounts must perform the additional function of isolating the mounted equipment from shock loads applied to the supporting structure. Designing the mounts for adequate shock isolation tends to compromise the primary function of acoustic and vibration isolation. For example, use of a low spring rate in the mounting system increases the acoustic and vibration isolation effectiveness. However, this low spring rate will result in correspondingly large deflections during shock events. Conventional mounts have a limited deflection range and may be damaged by shock events unless mitigating measures are taken. These measures include using a higher than optimum spring rate, placing dampers in parallel with the mounts, or using deflection-limiting devices such as xe2x80x9csnubbers,xe2x80x9d which stop motion by impact with a relatively hard material. All of these measures have disadvantages. Higher spring rates reduce acoustic and vibration isolation, additional dampers add weight and complexity, and snubbers produce shock energy on impact.
In some applications the acoustic and vibration mounts must operate effectively over a range of supported weight or with a range of angles to the vertical, or both. For example, submarine deck support mounts are subject to changing loads due to equipment and personnel movement. These deck support mounts must also operate effectively as the submarine changes angles. Designing the mounts for a range of supported weight and angles tends to compromise the primary function of acoustic and vibration isolation. Typically, the mount spring rate is increased to prevent excessive motion during load and angle changes, detrimental to the acoustic and vibration isolation performance of the mount.
Conventional air mounts generally have good acoustic performance, but typically operate at low pressure and therefore are quite large. High-pressure air mounts have been designed but provide reduced acoustic performance and a small deflection range. Conventional elastomeric mounts suffer from internal resonances that reduce acoustic performance and cannot compensate for changing loads or angles.
For the foregoing reasons there is a need for a mount that statically supports mounted equipment while isolating acoustic and vibration forces as well as shock loads to, or from, the mounted equipment. The mount should quickly damp the equipment response. The new mount should also be able to compensate for equipment weight and angle changes and allow for equipment height adjustment. Ideally, the new mount should be compact, which allows use of the mount in applications where the distance between the mounted component and support structure is limited.
Accordingly, it is an object of the present invention to provide a new mount that has superior performance in isolating acoustic and vibration forces as well as shock loads to or from a mounted component.
Another object of the present invention is to provide a new mount that dampens the amplification of the loads on the mounted component with negligible reduction of the isolation provided.
A further object of the present invention is to provide a new mount that is compact.
A still further object of the present is to provide a new mount that can compensate for equipment weight and angle changes.
Yet another object of the present invention is to provide a new mount that can provides for adjustment of the distance between the mounted component and the support structure.
According to the present invention, a mount is provided for support between first and second relatively movable members, such as a mounted component and a support structure, for reducing vibration and shock transmission between the members. The mount comprises a hollow cylindrical casing, within the casing a hollow cylindrical housing that is a piston shell, a piston disposed in and sealingly bonded to the piston shell, a compressible fluid such as air filling a variable volume chamber defined by the casing, the piston shell, and the piston, and an annular seal between the casing and the piston shell.
The casing has one closed end and one end with an opening, and is attached to the support structure. The piston shell is open at both ends and disposed for axial movement within the casing through the open end of the casing with the annular seal preventing fluid leakage between the casing and housing at the opening. A portion of the piston shell extends outwardly of the open end of the casing. The piston is axially movable relative to the piston shell by deformation of the bond material, which is resilient, and includes a piston rod that extends out of the piston shell and casing to attach to the mounted component.
The compressible fluid acts on the piston shell and piston to urge the piston shell and piston to an axial position relative to the casing. The static friction between the piston shell and annular seal and the stiffness of the bond material between the piston and the piston shell are selected so that only the piston moves relative to the casing in response to axial external forces on the movable members less than a predetermined net force for reducing vibration transmission between the members. The piston shell and piston move together relative to the casing when the static friction between the annular seal and the piston shell is overcome by axial external forces on the movable members greater than the predetermined net force for reducing vibration transmission between the members. A compressible fluid single-acting spring is thereby provided with large allowable relative movement of the support structure and the mounted component.
The present invention may further comprise a raised annular portion on the outer surface of the piston shell. The raised annular portion restricts the space between the outer surface of the piston shell and the inner surface of the casing to define a throttling passage, dividing the cavity into two variable volume chambers filled with a substantially incompressible fluid. The substantially incompressible fluid, such as oil, and an adjacent compressible fluid, such as air, fill inside of the piston shell up to the piston and resilient bond. To provide hydraulic communication through the wall of the piston shell, the piston shell has at least one opening between the raised annular portion and the end of the casing with the opening. A compressible fluid single-acting spring is thereby provided with large allowable relative movement of the mounted component and the support structure, and with damping provided by incompressible fluid flow through the throttling passage.
Also according to the present invention, an additional compressible fluid volume may be provided inside the casing but outside of the piston shell, on the side of the raised annular portion adjacent to the end of the casing with the opening. A double-acting spring is thereby provided.
Further according to the present invention, in addition to a first raised annular portion that creates a throttling passage for damping, a second raised annular portion on the outer surface of the piston shell is provided, disposed between the opening or openings in the piston shell wall and the compressible fluid that is outside of the piston shell. The second raised annular portion restricts the space between the outer surface of the piston shell and the inner surface of the casing to present a relatively restrictive passage for flow of the incompressible fluid, and divides the chamber between the first raised annular portion and the end of the casing with the opening into two chambers. Flow of incompressible fluid through this restrictive passage at a flow rate in excess of a predetermined flow rate substantially prevents hydraulic communication between the compressible fluid volumes inside and outside of the piston shell.
The second raised annular portion of the piston shell is designed to cause the mount to respond differently in cases of acoustic and vibration transmission, shock force transmission, and quasi-static force transmission. The type of force encountered by the mount determines the response characteristics of the two air springs. The compressible fluid spring inside the piston shell alone responds to acoustic and vibration forces. As the result of the flow restriction at the annular orifice, the chamber inside the piston shell is effectively decoupled from the remainder of the casing interior during shock, and each compressible fluid spring independently counteracts the shock force. The compressible fluid springs act together in response to large and relatively slow, quasi-static, forces.
The mount features a sliding seal for use as the annular seal between the casing and the piston shell, and an elastomeric material bonding the piston to the piston shell. A source of compressible fluid may be provided to vary the pressure of the compressible fluid springs, to compensate for changes in load or to adjust the length of the mount. By adding control methods, the mount could be made semi-active. Control methods could include use of a valved bypass to provide fluid communication between ends of the casing around the throttling passage, or use of electrorheological or magnetorheological fluid instead of oil, for example.
The material bonding the piston to the piston shell deforms to provide acoustic and vibration isolation, and the annular seal handles large forces and shock forces, while allowing side loads on the mount. A multi-axis mount system may be provided to include a plurality of uniaxial mounts as components. The mount is compact in size, which results in improved acoustic and vibration response and reduced system weight and cost. Analysis predicts superior performance in acoustic, vibration, and shock isolation over elastomeric-based passive mount systems. The combination of the annular seal and the material that bonds the piston to the piston shell provides both good acoustic and vibration isolation and reduced transmission of shock and other large forces in a compact size.